Boilers (e.g. those that are used in power generation) have a theoretical maximum thermal efficiency when the combustion is exactly stoichiometric. This will result in the best overall heat rate for the generator. However, in practice, boilers are run “lean”; i.e., excess air is used, which lowers flame temperatures and creates an oxidizing atmosphere which is conducive to slagging (further reducing thermal efficiency). Ideally the combustion process is run as close to stoichiometric as practical, without the mixture becoming too rich. A rich mixture is potentially dangerous by causing “backfires”. The objective is to control excess oxygen (XSO2) so that the CO will be continually on the “knee” of the CO vs. XSO2 curve.